The ball is snapped. The quarterback drops back, immediately surrounded by a chorus of grunts and groans, the sounds of linemen colliding. The play has just begun, but the pocket is already collapsing around him. He must focus his eyes downfield on his receivers and know where they're going while also reading the ­defense. Is that cornerback blitzing or dropping back? When will the safety leave the middle? The QB has fewer than three seconds to make sense of this mess. If he hesitates, even for a split second, he'll get sacked.

No other team sport is so dependent on the judgment of a single player, which is why NFL scouts and coaches take the decision-making skills of quarterbacks very seriously. Since the early 1970s, when Cowboys coach Tom Landry began using the Wonderlic intelligence test to evaluate potential Dallas players, the league has included it at the annual scouting combine, to assess every player entering the draft. Basically a short version of an IQ test, the Wonderlic is 12 minutes long and consists of 50 questions, which get progressively harder. The underlying assumption is that players with high scores (read: smarter) will make better decisions in the pocket. If a quarterback can solve pre-algebra problems quickly, then he'll be more likely to find his man while getting blitzed.

At first, this seems like a logical assumption. Just think of all the cognitive skills required to become a successful QB. He needs to memorize hundreds of offensive plays and dozens of defensive formations. He has to study game tape. And, in many instances, quarterbacks are responsible for changing the play at the line of scrimmage. This helps explain why NFL teams start to get nervous whenever the Wonderlic scores of a QB in the draft fall below 24, the unofficial average for the position. (In comparison, the average score for computer programmers is 29 while janitors score 15, a point below running backs.) Scouts believe a quarterback who isn't smart, at least by ...